1. Field of the Invention
The invention relates to refastenable pressure-sensitive adhesive closures of diapers and other articles such as envelopes, especially the target strips of the adhesive closures The invention is particularly concerned with the problem that such closures tend to lose their adhesiveness prematurely because of contamination.
2. Description of the Related Art
U.S. Pat. No. 4,163,077 (Antonsen et al.) attributes much of the present commercial success of disposable diapers to pressure-sensitive adhesive tape closure systems which replace safety pins, but says that contamination by talcum powder can greatly reduce the ability of the adhesive to adhere. The Antonsen patent answers this problem by using as the pressure-sensitive adhesive a rubbery block copolymer which is said to display good adhesion and shear properties even when contaminated with fine particulate material, e.g., talcum powder. In spite of the availability of such a pressure-sensitive adhesive, one of the most common complaints about disposable diapers is the failure of the adhesive tape closure, and it is believed that such failures primarily occur because of talcum powder contamination.
A refastenable diaper closure can be more resistant to contamination when it employs a more aggressive pressure-sensitive adhesive which when contaminated would lose some tackiness but still be sufficiently tacky to hold the diaper in place. If made too aggressive, a closure would be difficult to open when not contaminated, especially by persons who do not have strong fingers. There also is a hazard that the diaper might be torn upon opening an adhesive tape closure if adhesive is overly aggressive.
Other than the Antonsen patent, we are not aware of any prior teaching about how to minimize failures of refastenable adhesive diaper closures However, there are a number of publications about improving those closures in other respects. U.S. Pat. No. 4,237,889 (Gobran) concerns the problem that prior tape tabs would often tear, or if made not to tear, would either be too expensive or so stiff that a baby's tender skin might be injured. The Gobran patent answers this problem by using as the backing of the tab a crystalline polypropylene film having one smooth face and one specially textured face.
U.S. Pat. No. 4,436,520 (Lipko et al.) discloses that the outer or back sheet of a disposable diaper typically is embossed to reduce gloss, but that "the embossing materially reduces the adhesion of the embossed surface for the closure tabs so that the diaper is no longer secure in its use on the infant" (col. 1, lines 19-22). The Lipko patent further discloses that in embossed films of polyethylene having a (45.degree.) gloss of no greater than 8, the adhesion of the embossed surface is markedly enhanced when the surface has a mean value of maximum profile height of less than 150 and maximum peak to valley height of less than 230 as measured using a Surtronic 3 apparatus.
While the embossing height of the outer sheet of a disposable diaper typically is from 20 to 30 .mu.m, U.S. Pat. No 3,484,835 (Trounstine et al.) discloses a plastic film having a permanently embossed design which simulates a plain woven fabric for uses including the outer sheet of a disposable diaper where the embossing height is about 3 to 4 times the thickness of the film e.g., 75 to 100 .mu.m for a 25 .mu.m film.
U.S. Pat. No. 4,645,501 (Teed) points out that it is desirable to be able to unfasten and reposition the adhesive fastener tabs of disposable diapers but that because of the need for a strong bond between the fastener tab and the outer plastic sheet of the diaper, the fastener tab cannot generally be removed from the surface of the plastic sheet without tearing and pulling away the thin water-impervious plastic outer cover of the garment. To prevent the tearing of the outer cover, diapers often are made with a plastic reinforcing strip (sometimes called a "target strip") covering areas that can be contacted by the fastener tabs. The Teed patent avoids the need for a reinforcing strip while attaining repositionability by employing as the water-impervious plastic cover or outer sheet one that is embossed to have ridges or ribs that are separated by square recessed surface portions having downwardly tapering sides. In Example I, the plastic outer sheet is one mil (25 .mu.m) in thickness and has a recessed depth of 0.0023 inch (58 .mu.m).
Among publications concerning reinforcing or target strips for disposable diapers are U.S. Pat. No. 3,867,940 (Mesek et al.) which discloses a scrim reinforced backing sheet for disposable diapers; West German Offenlegungsschrift No. DE 3338201 Al (Molnlycke AB) discloses a nonelastic target strip of polyester; and European Patent Publication No. 0 080 647 A1 (Boussac Saint Freres) which discloses reinforced areas on the front of a disposable diaper, the reinforced areas being, for example, a smooth-surfaced film of polypropylene adhered to the outer sheet by a layer of adhesive.
In U.S. Pat. No. 4,643,730 (Chen et al.), a reinforcing strip is provided on the inner face of the outer or backing sheet of a disposable diaper by coating the sheet with a high-energy-radiation curable coating and curing the coating.
U.S. Pat. No. 4,543,139 (Freedman et al.) discloses refastenable adhesive closures for packages and particularly concerns the problem of such closures becoming ineffective due to contamination, mentioning oils and small food particles. The Freedman closure has two pressure-sensitive adhesive strata, one forming a strong bond to the body of a package and a second forming a relatively weak bond permitting it to be separated from the cover of the package. In the course of reclosing and reopening the package, the strong adhesive becomes mixed with the weak adhesive, thereby offsetting contamination encountered while the closure is open.
U.S. Pat. No. 4,655,761 (Grube et al.) discloses a disposable diaper with a refastenable tape system which utilizes a polypropylene tape material that is embossed on the outer surface and smooth on the adhesive-carrying surface. The adhesive on the tape is a semihard adhesive that provides high adhesion values to the polymer that nevertheless may be peeled away and refastened. The polymer forming the outer backing of the diaper is a relatively thick, embossed polymeric sheet.
U.S. Pat. No. 4,710,190 (Wood et al.) discloses a diaper having an improved reinforced area for receiving adhesive fastening tape. The diaper includes an outer liquid-impermeable film and a bilayer film bonded to the liquid-impermeable film as a peel-resistant reinforced fastening area. The bilayer film comprises a reinforcing layer and a room-temperature-nontacky bonding layer, the bonding layer holding the reinforcing layer to the liquid-impermeable film with greater force than that which the fastening tape applies when adhered to the top of bilayer film.
U.S. Pat. No. 4,397,905 (Dettmer et al.) discloses an adhesive tape which has a support film of a synthetic thermoplastic polymer and an adhesive layer on one side, the support film having a thickness of less than about 35 .mu.m and having provided on the nonadhesive surface elevations having a depth within a range of from about 5 to 20 .mu.m.
U.S. Pat. No. 4,536,362 (Donaldson) discloses a method for producing longitudinally ribbed plastic film which includes extruding through a pair of die lips, at least one of which is provided with slots which run at an angle to the direction of resin flow across the die lips. Impurities in the resin, such as gel particles, collect in the slots of the die lip and are extruded in the thickened, rib portion of the film. The ribs form strengthened areas which hinder the propagation of tears and punctures.
U.S. Pat. No. 4,349,598 (White) discloses a retroreflective film having an array of light-reflecting right triangle prisms between a transparent surface layer and a backing layer. U.S. Pat. No. 4 588 258 (Hoopman) discloses cube-corner retroreflective articles having improved angularity along multiple viewing planes, the articles comprising at least one matched pair of cube-corner retroreflective elements which are rotated 180.degree. with respect to one another, the three lateral mutually perpendicular faces of the elements being defined at their bases by linear edges that lie in a common plane, and the optical axes of the elements being tilted toward one another.
U.S. Pat. No. 4,576,850 (Martens) discloses an article comprising a shaped, plastic, monolithic layer, or body, comprising crosslinked polymer with hard and soft segments of moieties and having a microstructure-bearing surface which is prepared by a process comprising filling a mold master, bearing or encoded with the microstructure to be replicated, with a fluid, castable, one-part, radiation addition-polymerizable crosslinkable, synthetic, organic oligomeric composition having hard segments and soft segments, exposing the resulting cast composition to radiation, and thereby forming the article. The article may be, for example, a retroreflective cube-corner sheeting, Fresnel lens or video disc.
U.S. Pat. No. 4,476,593 (Fanselow et al.) discloses a tanning blanket having a plurality of incremental reflectors in a Fresnel pattern on a flexible substrate, which reflectors reflect incident solar radiation toward the person and distribute the reflected radiation across the flanks of the person.
European Patent Publication No. 0 205 289 Minnesota Mining and Manufacturing Company) discloses a conformable drag reduction article, typically a film, having a patterned surface capable of reducing drag resistance by fluid flowing thereover on a first side and an adhesive on a second side, the second side being parallel to the first side.
Refastenable pressure-sensitive adhesive closures are also widely used on envelopes such as are commonly used for interoffice mail. Although such envelopes may be designed to be used up to about 30 times before being discarded, their closures tend to become contaminated and to lose their adhesiveness in less than ten mailings